1. Field of the Invention
The present invention relates to an anode useful in an electrochemical cell used for the direct production of essentially dry halogen gas from essentially anhydrous halogen halide, or for a process for such direct production of essentially dry halogen gas. This cell or process may be used to produce halogen gas such as chlorine, bromine, fluorine and iodine from a respective anhydrous hydrogen halide, such as hydrogen chloride, hydrogen bromide, hydrogen fluoride and hydrogen iodide. In particular, the anode of the present invention comprises the oxides of the elements tin, germanium and lead and mixtures comprising at least one of the respective oxides of such elements.
2. Description of the Related Art
A number of commercial processes have been developed to convert HCl into usable chlorine gas. See e.g., F. R. Minz, "HCl-Electrolysis--Technology for Recycling Chlorine", Bayer AG, Conference on Electrochemical Processing, Innovation & Progress, Glasgow, Scotland, UK Apr. 21-23, 1993. The current commercial electrochemical process is known as the Uhde process. In the Uhde process, aqueous HCl solution of approximately 22% is fed at 65.degree. to 80.degree. C. to both compartments of an electrochemical cell, where exposure to a direct current in the cell results in an electrochemical reaction and a decrease in HCl concentration to 17% with the production of chlorine gas and hydrogen gas. A polymeric separator divides the two compartments. The process requires recycling of dilute (17%) HCl solution produced during the electrolysis step and regenerating an HCl solution of 22% for feed to the electrochemical cell. The overall reaction of the Uhde process is expressed by the equation: EQU 2HCl (aqueous).fwdarw.H.sub.2 (wet)+Cl.sub.2 (wet) (1)
As is apparent from equation (1), the chlorine gas produced by the Uhde process is wet, usually containing about 1% to 2% water. This wet chlorine gas must then be further processed to produce a dry, usable gas. If the concentration of HCl in the water becomes too low, it is possible for oxygen to be generated from the water present in the Uhde process. This possible side reaction of the Uhde process due to the presence of water is expressed by the equation: EQU 2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+ +4e.sup.- (2)
Further, the presence of water in the Uhde system limits the current densities at which the cells can perform to less than 500 amps./ft..sup.2, because of this side reaction. The result is reduced electrical efficiency and corrosion of the cell components due to the oxygen generated.
Another electrochemical process for processing aqueous HCl has been described in U.S. Pat. No. 4,311,568 to Balko. Balko employs an electrolytic cell having a solid polymer electrolyte membrane. Hydrogen chloride, in the form of hydrogen ions and chloride ions in aqueous solution, is introduced into an electrolytic cell. The solid polymer electrolyte membrane is bonded to the anode to permit transport from the anode surface into the membrane. In Balko, controlling and minimizing the oxygen evolution side reaction is an important consideration. Evolution of oxygen decreases cell efficiency and leads to rapid corrosion of components of the cell. The design and configuration of the anode pore size and electrode thickness employed by Balko maximizes transport of the chloride ions. This results in effective chlorine evolution while minimizing the evolution of oxygen, since oxygen evolution tends to increase under conditions of chloride ion depletion near the anode surface. In Balko, although oxygen evolution may be minimized, it is not eliminated. As can be seen from FIGS. 3 to 5 of Balko, as the overall current density is increased, the rate of oxygen evolution increases, as evidenced by the increase in the concentration of oxygen found in the chlorine produced. Balko can run at higher current densities, but is limited by the deleterious effects of oxygen evolution. If the Balko cell were to be run at high current densities, the anode would be destroyed.
In general, the rate of an electrochemical process is characterized by its current density. In many instances, a number of electrochemical reactions may occur simultaneously. When this is true, the electrical driving force for electrochemical reactions is such that it results in an appreciable current density for more than one electrochemical reaction. For these situations, the reported or measured current density is a result of the current from more than one electrochemical reaction. This is the case for the electrochemical oxidation of aqueous hydrogen chloride. The oxidation of the chloride ions is the primary reaction. However, the water present in the aqueous hydrogen chloride is oxidized to evolve oxygen as expressed in equation (2). This is not a desirable reaction. The current efficiency allows one to describe quantitatively the relative contribution of the current from multiple sources. For example, if at the anode or cathode multiple reactions occur, then the current efficiency can be expressed as: ##EQU1## where .eta..sub.j is the current efficiency of reaction j, and where there are NR number of reactions occurring.
For the example of an aqueous solution of HCl and an anode, the general expression above is: ##EQU2##
In the specific case of hydrogen chloride in an aqueous solution, oxidation of chloride is the primary reaction, and oxygen evolution is the secondary reaction. In this case, the current density is the sum of the two anodic reactions. Since .eta..sub.o.sbsb.2 is not zero, the current efficiency for chloride oxidation is less than unity, as expressed in equations (6) and (7) below. Whenever one is concerned with the oxidation of chloride from an aqueous solution, then the current efficiency for oxygen evolution is not zero and has a deleterious effect upon the yield and production of chlorine. ##EQU3##
Furthermore, electrolytic processing of aqueous HCl can be mass-transfer limited. Mass-transfer of species is very much influenced by the concentration of the species as well as the rate of diffusion. The diffusion coefficient and the concentration of species to be transported are important factors which affect the rate of mass transport. In an aqueous solution, such as that used in Balko, the diffusion coefficient of a species is .about.10.sup.-5 cm..sup.2 /sec. In a gas, the diffusion coefficient is dramatically higher, with values .about.10.sup.-2 cm..sup.2 /sec. In normal industrial practice for electrolyzing aqueous hydrogen chloride, the practical concentration of hydrogen chloride or chloride ion is .about.17% to 22%, whereas the concentration of hydrogen chloride is 100% in a gas of anhydrous hydrogen chloride. Above 22%, conductance drops, and the power penalty begins to climb. Below 17%, oxygen can be evolved from water, per the side reaction of equation (2), corroding the cell components, reducing the electrical efficiency, and contaminating the chlorine.
Electrochemical cells for converting aqueous HCl to chlorine gas by passage of direct electrical current through the solution are also known. Electrochemical cells for processing aqueous HCl, as exemplified by U.S. Pat. No. 4,210,501 to Dempsey et al., have typically used one or more reduced oxides of platinum group metals, such as ruthenium, iridium or platinum, or one or more reduced oxides of a valve metal, such as titanium, tantalum, niobium, zirconium, hafnium, vanadium or tungsten to stabilize the electrodes against oxygen, chlorine and generally harsh electrolysis conditions. U.S. Pat. No. 4,959,132 to Fedkiw discloses a process for producing an electrochemically active film proximate a solid polymer electrolyte membrane which may be used in electrochemical reactions, e.g., chloralkali processes. Fedkiw's process involves exposing a metal ion-loaded polymer membrane to a chemical reductant which reduces the ions to metal (0) state and produces an electrochemically active film. Tin sulfate, SnSO.sub.4, is disclosed as the chemical reductant in the deposition of platinum as the electrochemically active film. Fedkiw also discloses the production of an electrocatalytic single metal film of lead, the production of films of alloys, which include tin/platinum, and the production of films of mixed metal composition, including lead/platinum, lead/palladium and lead/silver. However, Fedkiw does not recognize that the oxides of tin, germanium and lead and various mixtures comprising at least one of these oxides have applicability to the electrochemical processing of anhydrous hydrogen halides, with resulting high current densities.